1. Field of the Invention
The present invention concerns a telephone handset and relates to the earpiece part of the handset. The aim of the invention is to reduce the thickness of the handset in the area of the earpiece part.
The invention applies in particular to mobile telephone handsets. It is particularly necessary to reduce the volume and more particularly the thickness of these devices since they are intended to be carried in the user's pocket.
2. Description of the Prior Art
FIG. 1 shows the earpiece part of a prior art telephone handset. It comprises a housing 1 in which an earpiece capsule 2 is mounted.
The earpiece capsule essentially comprises a piezo-electric (ceramic) disk 3 glued to a metal disk 4.
Alternating electric voltage variations applied to the terminals of this assembly generate alternating vibration of the assembly at the same frequency as the voltage variations and therefore produce sound pressure waves. However, the response curve of the system representing the amplitude of the sound vibrations as a function of the excitation frequency has a peak corresponding to its natural frequency (around 1 000 Hz) and high attenuation beyond that frequency.
To obtain an acceptable response curve in the range of voice frequencies, it is necessary to damp the natural frequency and to amplify high frequencies.
An acoustic resistance is therefore placed behind he vibrating assembly 3, 4 to damp the peak and a Helmoltz resonator is placed in front of this assembly to amplify high frequencies.
To this end, the disk 4 is clamped between two parts 5 and 6 forming a capsule 2 and there is a hole 7 through the rear wall 6 covered by a fabric 8 the role of which is to constitute micro-holes creating an acoustic resistance.
The Helmoltz resonator for amplifying high frequencies consists of the volume 9 behind the disk 4 and the part 5 and a few holes 10 having a diameter in the order of 1 mm in the front wall of the part 5.
To avoid disrupting the operation of the acoustic resistance it is necessary to leave a gap x of 1 mm to 2 mm behind the rear wall of the part 6 of the capsule 2.
The printed circuit board 11 carrying the electronic components 12 of the handset therefore cannot be at a distance less than this value x from the rear wall of the capsule 2.
A circular gasket having a thickness e of approximately 1 mm is disposed between the earpiece capsule 2 and the wall 1 of the housing.
An aim of the present invention is to propose a telephone handset in which the earpiece part is thinner. The invention therefore provides a structural arrangement enabling this thickness to be reduced.